Ldmos transistor and method of forming the ldmos transistor with improved rds*cgd

ABSTRACT

The Rds*Cgd figure of merit (FOM) of a laterally diffused metal oxide semiconductor (LDMOS) transistor is improved by forming the drain drift region with a number of dopant implants at a number of depths, and forming a step-shaped back gate region with a number of dopant implants at a number of depths to adjoin the drain drift region.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a division of U.S. patent application Ser. No.16/666,587, which is a continuation of U.S. patent application Ser. No.15/244,616, which is a continuation of U.S. patent application Ser. No.14/556,185, which claims benefits of and priority to ProvisionalApplication No. 61/948,853 filed on Mar. 6, 2014. The entirety of eachof the above-referenced applications is hereby incorporated by referenceherein.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The disclosure relates to LDMOS transistors and, more particularly, butnot exclusively, to an LDMOS transistor and a method of forming theLDMOS transistor with improved Rds*Cgd.

2. Description of the Related Art

A metal oxide semiconductor (MOS) transistor is a well-knownsemiconductor device that has a source, a drain, a body which has achannel region that lies between and touches the source and drain, and agate that lies over and is isolated from the channel region by a gatedielectric layer. There are two types of MOS transistors: an NMOStransistor that has n+ source and drain regions with a p-type channelregion, and a PMOS transistor that has p+ source and drain regions withan n-type channel region.

In operation, when the source and the body are grounded, a positivevoltage is placed on the drain to set up a drain-to-source electricfield, and a voltage is placed on the gate that is greater than athreshold voltage, a current flows from the drain to the source. Whenthe voltage placed on the gate is less than the threshold voltage, suchas when the gate is pulled down to ground, no current flows.

Current-generation MOS transistors are commonly used in low-voltageenvironments that range from, for example, 1.2V to 5V. In contrast, ahigh-voltage MOS transistor is a transistor that operates with voltagesin the range of, for example, 10V to 400V. In order to handle the highervoltages, high-voltage MOS transistors are bigger than low-voltage MOStransistors.

One type of high-voltage MOS transistor is known as a laterally diffusedMOS (LDMOS) transistor. LDMOS transistors are MOS transistors that alsohave a drain drift region. The drain drift region, which touches andlies between the drain and the channel region, has the same conductivitytype as the drain, but a lower dopant concentration than the drain. Inoperation, the drain drift region reduces the magnitude of thedrain-to-source electric field.

A new figure of merit (FOM) for high current (e.g., 10 A and above) andhigh frequency (1-10 MHz and higher) LDMOS transistors is Rds*Cgd, whichis the product of the drain-to-source resistance (Rds) and thegate-to-drain capacitance (Cgd). To improve this FOM, it is desirable toreduce the Rds value, the Cgd value, or both of the values.

One approach to reducing Cgd is to use split or step gates in lieu ofone gate. With step or split gates, a main gate and, for example, twoprogressively thinner gates are used so that the closer a gate lies tothe drain region the thicker the underlying gate dielectric layer. Onedrawback to this approach, however, is that split or step gates aredifficult and expensive to fabricate. In addition, split or step gatescan require longer drain drift regions, which limit the device inhigh-speed mobile applications due to an increased Rds.

SUMMARY OF THE INVENTION

One implementation includes a transistor, e.g. an LDMOS transistor thatprovides improved Rds*Cgd. The transistor includes a p-type substratehaving a top surface and including a p-buried layer (PBL). An n-typesource region and an n-type drain region are located in the substrateand a gate electrode is positioned between the source and drain regions.

An n-type drain drift region located between the PBL and the drainregion extends under a first portion of the gate electrode. A p-typebody region between the PBL and the source region extends under a secondportion of the gate electrode. A p-type intermediate region is locatedbetween the PBL and the p-type body region. The p-type body regionextends laterally from under the source toward the drain by a firstdistance, and the p-type intermediate region extends laterally fromunder the source toward the drain by a second distance greater than thefirst distance.

Another implementation includes a method of forming an integratedcircuit, e.g. having an LDMOS transistor that provides improved Rds*Cgd.The method includes forming a source region and a drain region having afirst conductivity type in a semiconductor substrate having a secondopposite conductivity type. A gate is formed over a top surface of thesubstrate between the source region and the drain region. A drain driftregion is formed under the drain region and partially under the gate.The drain drift region has the first conductivity type and extends fromthe drain region toward the source region. The drain drift regionincludes a first drain drift sub-region (DDSR) near the surface thatextends toward the source region a first distance. The drain driftregion further includes a second DDSR below the first DDSR that extendstoward the source region a second distance less than the first distance.The drain drift region further includes a third DDSR between the firstDDSR and the second DDSR that extends toward the source region a thirddistance greater than the second distance and less than the firstdistance.

A better understanding of the features and advantages of variousexamples will be obtained by reference to the following detaileddescription and accompanying drawings which set forth an illustrativeembodiment in which the principals of the invention are utilized.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view illustrating an example of a LDMOStransistor 100 in accordance with various examples.

FIGS. 2A-2G are cross-sectional views illustrating an example of amethod 200 of forming a LDMOS transistor structure in accordance withvarious examples.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a cross-sectional view that illustrates an example of aLDMOS transistor 100 in accordance with an example implementation. Asdescribed in greater detail below, LDMOS transistor 100 improves theRds*Cgd by utilizing multiple implants in both the drain drift regionand an adjoining step-shaped back gate region.

As shown in FIG. 1 , LDMOS transistor 100 includes a semiconductormaterial 110, such as a substrate or an epitaxial layer, and a draindrift region 112 that lies within semiconductor material 110. Draindrift region 112 has a first conductivity type and two horizontal dopantconcentration peaks: a first peak at a depth D1 measured a distance downfrom a top surface 114 of semiconductor material 110, and a second peakat a depth D2 measured a distance down from the depth D1. In the presentexample, drain drift region 112 has an n conductivity type.

The depth D1 defines a drift top section 120 that extends from the topsurface 114 of semiconductor material 110 down to the depth Dl. Drifttop section 120 has a dopant concentration profile where the dopantconcentration increases with increasing depth. In the present example,drift top section 120 continuously increases from a low dopantconcentration at the top surface 114 of semiconductor material 110 to ahigh dopant concentration at the depth Dl. Further, the largest dopantconcentration within drift top section 120 is at the depth D1.

The depth D1 and the depth D2 define a drift middle section 124 thatextends from the depth D1 down to the depth D2. Drift middle section 124has a dopant concentration profile where the dopant concentration firstdecreases with increasing depth, and then increases with increasingdepth.

In the present example, drift middle section 124 continuously decreasesfrom a high dopant concentration at depth D1 to a lower dopantconcentration at a point between the depths D1 and D2, and thencontinuously increases to a higher dopant concentration at depth D2.Further, the two largest dopant concentrations within drift middlesection 124 are at the depths D1 and D2.

The depth D2 also defines a drift bottom section 126 that extends down adistance from the depth D2. Drift bottom section 126 has a dopantconcentration profile where the dopant concentration decreases withincreasing depth from the depth D2. In the present example, drift bottomsection 126 continuously decreases from a high dopant concentration atdepth D2 to a lower dopant concentration. Further, the largest dopantconcentration within drift bottom section 126 is at the depth D2.

As further shown in FIG. 1 , LDMOS transistor 100 also includes a backgate region 128 that lies within semiconductor material 110 to touchdrain drift region 112. Back gate region 128 has a second conductivitytype, and a step shape that corresponds with three horizontal dopantconcentration peaks of the same conductivity type: a peak at a depth D3measured a distance down from the top surface 114 of semiconductormaterial 110, a peak at a depth D4 measured a distance down from thedepth D3, and a peak at a depth D5 measured a distance down from thedepth D4. In the present example, back gate region 128 has a pconductivity type.

The depth D3 defines a back gate top section 130 that extends from thetop surface 114 of semiconductor material 110 down to the depth D3. Backgate top section 130 has a dopant concentration profile where the dopantconcentration increases with increasing depth. In the present example,back gate top section 130 continuously increases from a low dopantconcentration at the top surface 114 of semiconductor material 110 to ahigh dopant concentration at the depth D3. Further, the largest dopantconcentration within back gate top section 130 is at the depth D3.

The depths D3 and D4 also define a back gate middle section 134 thatextends from the depth D3 down to the depth D4. Back gate middle section134 has a dopant concentration profile where the dopant concentrationfirst decreases with increasing depth, and then increases withincreasing depth.

In the present example, back gate middle section 134 continuouslydecreases from a high dopant concentration at depth D3 to a lower dopantconcentration at a point between the depths D3 and D4, and thencontinuously increases to a higher dopant concentration at depth D4.Further, the two largest dopant concentrations within back gate middlesection 134 are at the depths D3 and D4.

The depth D4 and the depth D5 define a back gate middle section 136 thatextends from the depth D4 down to the depth D5. Back gate middle section136 has a dopant concentration profile where the dopant concentrationfirst decreases with increasing depth, and then increases withincreasing depth.

In the present example, back gate middle section 136 continuouslydecreases from a high dopant concentration at depth D4 to a lower dopantconcentration at a point between the depths D4 and D5, and thencontinuously increases to a higher dopant concentration at depth D5.Further, the two largest dopant concentrations within back gate middlesection 136 are at the depths D4 and D5.

The depth D5 further defines a back gate bottom section 138 that extendsdown a distance from the depth D5. Back gate bottom section 138 has adopant concentration profile where the dopant concentration decreaseswith increasing depth from depth D5. In the present example, back gatebottom section 138 continuously decreases from a high dopantconcentration at depth D5 to a lower dopant concentration. Asillustrated, the depth D3 lies between the depth D1 and the depth D2. Inaddition, the depth D4 lies below the depth D2. Further, a portion ofback gate middle section 136 and back gate bottom section 138 of backgate region 128 lies directly below drain drift region 112.

As additionally shown in FIG. 1 , LDMOS transistor 100 includes a pairof shallow trench isolation (STI) regions 140 that lie withinsemiconductor material 110. The STI regions 140 have a lower surface 142that lies below the depth D1. In the present example, the STI regions140 also have a top surface that lies substantially in the same plane asthe top surface 114 of semiconductor material 110.

LDMOS transistor 100 further includes a drain region 150, a sourceregion 152, and a surface region 154 that each lie within semiconductormaterial 110. Drain region 150, which has the first conductivity type,lies between the STI regions 140 to touch drain drift region 112. Drainregion 150 has a dopant concentration substantially greater than ahighest dopant concentration of drain drift region 112. In the presentexample, drain region 150 is implemented as an n+region.

Source region 152, which also has the first conductivity type, touchesback gate region 128. Source region 152 has a dopant concentrationsubstantially equal to the dopant concentration of drain region 150. Inthe present example, source region 152 is implemented as an n+ region.

Surface region 154, which further has the first conductivity type,touches the top surface 114 of semiconductor material 110, back gateregion 128, and source region 152, and lies directly above a portion ofback gate region 128. Surface region 154 has a dopant concentrationsubstantially greater than a highest dopant concentration of drain driftregion 112. In the present example, surface region 154 is implemented asan n+ region. (Surface region 154 can optionally be omitted.)

LDMOS transistor 100 additionally includes a body contact region 156that lies within semiconductor material 110 to touch back gate region128. Body contact region 156 has the second conductivity type, and adopant concentration substantially greater than a highest dopantconcentration of back gate region 128. In the present example, bodycontact region 156 is implemented as a p+ region.

As also shown in FIG. 1 , LDMOS transistor 100 includes a gatedielectric layer 160 that touches the top surface 114 of semiconductormaterial 110, a gate 162 that touches and lies over gate dielectriclayer 160, and sidewall spacers 164 that touch and laterally surroundgate 162. Back gate region 128 includes a channel region 166 that liesbetween and touches drain drift region 112 and source region 152. Gate162, in turn, lies directly over drain drift region 112 and the channelregion 166 of back gate region 128.

In operation, when source region 152 and body contact region 156 aregrounded, a positive voltage, such as 16V, is placed on drain region 150to set up a drain-to-source electric field, and a voltage is placed ongate 162 that is greater than a threshold voltage, a current flows fromdrain region 150 to source region 152. When the voltage placed on gate162 is less than the threshold voltage, such as when gate 162 is pulleddown to ground, no current flows.

One of the advantages of implementations consistent with the disclosureis that the region of drift top section 120 at and immediately below thetop surface 114 of semiconductor material 110 has a light dopantconcentration that increases with depth. By utilizing a region of lightdopant concentration at and near the top surface 114 of semiconductormaterial 110, the gate-to-drain capacitance Cgd can be reduced which, inturn, improves the Rds*Cgd. Another advantage of implementationsconsistent with the disclosure is that the higher dopant concentrationsat the depths D1 and D2 reduce the drain-to-source resistance Rds, whichfurther improves the Rds*Cgd.

In addition, drain drift region 112 continues to reduce the magnitude ofthe drain-to-source electric field due to the presence of the lowerdopant concentration regions within drift top section 120 and driftmiddle section 124. The drain-to-source resistance Rds to thedrain-to-source breakdown voltage (BV) is best traded off by alsoutilizing the interaction between drift bottom section 126 and back gatemiddle section 136 to balance the charge at the horizontal region ofhigh dopant concentration at the depth D2.

A further advantage of implementations consistent with the disclosure isthat the horizontal region of high dopant concentration at the depth D1that lies below gate 162 is relatively large which, in turn, reduces thechannel resistance and the JFET resistance. The JFET resistance is theresistance associated with a subsurface region adjacent to the channel166 where the width of the depletion region varies with the voltages ondrain 150 and gate 162.

In addition, the surface at the horizontal regions of high dopantconcentration at the depths D1 and D2 are easily depleted for a reducedCgd. Further, at increased drain voltage, the step shape area of thehorizontal regions of high dopant concentrations at the depths D3 and D4can screen the increasing electric field at the horizontal regions ofhigh dopant concentration at the depths D1 and D2 that lie below gate162. This phenomenon works together with the charge balance betweendrain drift region 112 and back gate region 128 to increase the devicedrain-to-source breakdown voltage, or, at a targeted devicedrain-to-source breakdown voltage, the drain drift region 112 length(drift region underneath the lower surfaces 142 of the STI regions 140)can be reduced for a reduced Rds which, in turn, improves the totalRds*Cgd. Thus, implementations consistent with the disclosure mayimprove the Rds*Cgd by reducing both the Rds and the Cgd values.

FIGS. 2A-2G show a series of cross-sectional views that illustrate anexample of a method 200 of forming a LDMOS transistor structure inaccordance with implementations consistent with the disclosure. Method200 utilizes a conventionally-formed semiconductor material 210, such asa substrate or an epitaxial layer.

Method 200 begins by forming a drain drift region 212 withinsemiconductor material 210. Drain drift region 212 has a firstconductivity type and two horizontal dopant concentration peaks: a firstpeak at a depth D1 measured down a distance from a top surface 214 ofsemiconductor material 210, and a second peak at a depth D2 measureddown a distance from depth D1. In the present example, drain driftregion 212 is formed to have an n conductivity type.

Drain drift region 212 can be formed by first forming a patternedphotoresist layer 216 on semiconductor material 210. Patternedphotoresist layer 216 is formed in a conventional manner, which includesdepositing a layer of photoresist, projecting a light through apatterned black/clear glass plate known as a mask to form a patternedimage on the layer of photoresist, and removing the imaged photoresistregions, which were softened by exposure to the light.

After patterned photoresist layer 216 has been formed, dopants areimplanted into semiconductor material 210 through patterned photoresistlayer 216 to form an upper region 220. Upper region 220 has a horizontaldopant concentration peak at the depth Dl. In the present example,arsenic is implanted to form an n-type upper region 220. The arsenicdopants can be implanted with, for example, a dose of 4×10¹² to 8×10¹²and an energy of 200 keV to 350 keV.

With patterned photoresist layer 216 still in place, dopants are againimplanted into semiconductor material 210 through patterned photoresistlayer 216, this time to form a lower region 222. Lower region 222 has ahorizontal dopant concentration peak at the depth D2. In the presentexample, phosphorous is implanted to form an n-type lower region 222.The phosphorous dopants can be implanted with, for example, a dose of8×10¹² to 2×10¹³ and an energy of 100 keV to 400 keV.

After lower region 222 has been formed, patterned photoresist layer 216is removed in a conventional manner, such as with an ash process.Following this, a thermal drive process diffuses and activates thedopants to complete the formation of drain drift region 212. The thermaldrive process can include a heat treatment of 1100° C. for 90 minutes orequivalent conditions, for example, 1125° C. for 50 minutes, or 1050° C.for 270 minutes.

The depth D1 defines a drift top section 224 that extends from the topsurface 214 of semiconductor material 210 down to the depth Dl. Portionsof drift top section 224 are doped during the thermal drive process,which causes dopants from upper region 220 to out diffuse up into drifttop section 224.

Drift top section 224 has a dopant concentration profile where thedopant concentration increases with increasing depth. In the presentexample, drift top section 224 continuously increases from a low dopantconcentration at the top surface 214 of semiconductor material 210 to ahigh dopant concentration at the depth D1. Further, the largest dopantconcentration within drift top section 224 is at the depth D1.

The depth D1 and the depth D2 define a drift middle section 226 thatextends from the depth D1 down to the depth D2. Portions of drift middlesection 226 are doped during the thermal drive process, which causesdopants from upper region 220 to out diffuse down, and portions of lowerregion 222 to out diffuse up into drift middle section 226.

Drift middle section 226 has a dopant concentration profile where thedopant concentration first decreases with increasing depth, and thenincreases with increasing depth. In the present example, drift middlesection 226 continuously decreases from a high dopant concentration atdepth D1 to a lower dopant concentration at a point between the depthsD1 and D2, and then continuously increases to a higher dopantconcentration at depth D2. Further, the two largest dopantconcentrations within drift middle section 226 are at the depths D1 andD2.

The depth D2 also defines a drift bottom section 228 that extends down adistance from the depth D2. Drift bottom section 228 is doped during thethermal drive process, which causes dopants from lower region 222 to outdiffuse down into bottom section 228. (The order in which the upper andlower regions 220 and 222 are formed can alternately be reversed.)

Drift bottom section 228 has a dopant concentration profile where thedopant concentration decreases with increasing depth from depth D2. Inthe present example, drift bottom section 228 continuously decreasesfrom a high dopant concentration at depth D2 to a lower dopantconcentration. Further, the largest dopant concentration within driftbottom section 228 is at the depth D2.

As shown in FIG. 2B, after drain drift region 212 has been formed, apair of shallow trench isolation (STI) regions 230 is formed insemiconductor material 210 to touch drain drift region 212. The STIregions 230 can be formed in a conventional manner. For example, a hardmask can be formed over semiconductor material 210. After the hard maskhas been formed, semiconductor material 210 is etched through the hardmask to form a number of trenches in semiconductor material 210. Next,the hard mask is removed, and a non-conductive material is deposited onthe top surface of semiconductor material 210 to fill up the trenches.The non-conductive material on the top surface of semiconductor material210 is then removed, such as with a chemical-mechanical planarization(CMP) process, to leave the STI regions 230 in the trenches.

As further shown in FIG. 2B, after the STI regions 230 have been formed,a doped region 232 is next formed within semiconductor material 210. Thedoped region 232 has a back gate region 234 of the second conductivitytype, and a surface region 236 of the first conductivity type thattouches back gate region 234.

Back gate region 234 is formed to have a step shape that correspondswith three dopant concentration peaks: a peak at a depth D3 down fromthe top surface of semiconductor material 210, a peak at a lower depthD4, and a peak at a yet lower depth D5. In the present example, backgate region 234 has a p conductivity type, and surface region 236 has an conductivity type.

Back gate region 234 can be formed by first blanket implanting dopantsinto semiconductor material 210 to form a buried region 240 that touchesand lies below the bottom section 228 of drain drift region 212. Buriedregion 240 has a dopant concentration peak at the depth D5. In thepresent example, boron is implanted to form a p-type buried region 234.The boron dopants can be implanted with, for example, a dose of 1×10¹²to 9×10¹³ and an energy of 400 keV to 900 keV.

As shown in FIG. 2C, after buried region 240 has been formed, apatterned photoresist layer 242 is conventionally formed onsemiconductor material 210. After patterned photoresist layer 242 hasbeen formed, dopants are angle implanted into semiconductor material 210through patterned photoresist layer 242 to form an intermediate region244. Intermediate region 244 has a dopant concentration peak at thedepth D4. In the present example, boron is implanted to formintermediate region 244. The boron dopants can be implanted with, forexample, a dose of 2×10¹³ to 4×10¹³ and an energy of 300 keV to 600 keV.

With patterned photoresist layer 242 still in place, dopants are againimplanted into semiconductor material 210 through patterned photoresistlayer 242 to form a body region 246. Body region 246 has a dopantconcentration peak at the depth D3. In the present example, boron isimplanted to form body region 246. The boron dopants can be implantedwith, for example, a dose of 5×10¹³ to 3×10¹⁴ and an energy of 70keV to300 keV.

After body region 246 has been formed, dopants are yet again implantedinto semiconductor material 210 through patterned photoresist layer 242to reduce the size of back gate region 234 and form surface region 236.Surface region 236 touches the top surface 214 of semiconductor material210 and lies above body region 246. In the present example, arsenic isimplanted to form surface region 236. The arsenic dopants can beimplanted with, for example, a dose of 5×10¹³ to 1×10¹⁵ and an energy of30 keV to 160 keV. (The formation of surface region 236 can optionallybe omitted.)

After the implant, patterned photoresist layer 242 is removed in aconventional fashion. Following this, a thermal drive process isperformed to diffuse and activate the dopants, and complete theformation of doped region 232, back gate region 234, and surface region236. In the present example, surface region 236 and the immediatelysurrounding area have an n-type conductivity following the thermaldrive, while back gate region 234 has a p-type conductivity followingthe thermal drive. (The order in which the drain drift region 212 anddoped region 232 are formed can alternately be reversed.)

The depth D3 defines a substrate top section 250 that extends from thetop surface 114 of semiconductor material 110 down to the depth D3.Substrate top section 250 has a dopant concentration profile below andadjacent to surface region 236 where the dopant concentration increaseswith increasing depth. In the present example, substrate top section 250continuously increases from a low dopant concentration below andadjacent to surface region 236 to a high dopant concentration at thedepth D3. Further, the largest dopant concentration within substrate topsection 250 is at the depth D3.

The depth D3 and the depth D4 define a substrate middle section 252 thatextends from the depth D3 down to the depth D4. Substrate middle section252 has a dopant concentration profile where the dopant concentrationfirst decreases with increasing depth, and then increases withincreasing depth.

In the present example, substrate middle section 252 continuouslydecreases from a high dopant concentration at depth D3 to a lower dopantconcentration at a point between the depths D3 and D4, and thencontinuously increases to a higher dopant concentration at depth D4.Further, the two largest dopant concentrations within substrate middlesection 252 are at the depths D3 and D4.

The depth D4 and the depth D5 define a substrate middle section 254 thatextends from the depth D4 down to the depth D5. Substrate middle section254 has a dopant concentration profile where the dopant concentrationfirst decreases with increasing depth, and then increases withincreasing depth.

In the present example, substrate middle section 254 continuouslydecreases from a high dopant concentration at depth D4 to a lower dopantconcentration at a point between the depths D4 and D5, and thencontinuously increases to a higher dopant concentration at depth D5.Further, the two largest dopant concentrations within substrate middlesection 254 are at the depths D4 and D5.

The depth D5 also defines a substrate bottom section 256 that extendsdown a distance from the depth D5. Substrate bottom section 256 has adopant concentration profile where the dopant concentration decreaseswith increasing depth from depth D5. In the present example, substratebottom section 256 decreases from a high dopant concentration at depthD5 to a lower dopant concentration. As illustrated, the depth D3 liesbetween the depth D1 and the depth D2. In addition, the depth D4 liesbelow the depth D2. Further, a portion of back gate region 234 of thesecond (p) conductivity type lies directly below drain drift region 112.

As shown in FIG. 2D, once doped region 232 has been formed, method 200next forms a gate dielectric layer 260 on the top surface 214 ofsemiconductor material 210. A cleanup etch of, for example, a wet etchusing dilute hydrofluoric acid, can be performed prior to forming gatedielectric layer 260 to remove any unwanted oxide on the top surface ofsemiconductor material 210.

Gate dielectric layer 260 can be implemented with a thermally grownsilicon dioxide, and have a thickness that varies according to thevoltages to be used. For example, gate dielectric layer 260 can have12-15 nm of thermally grown silicon dioxide to support 5V gateoperation. Gate dielectric layer 260 can include additional layers ofother dielectric material such as silicon oxynitride or hafnium oxide.

Following this, a layer of gate material 262 is deposited on gatedielectric layer 260. The layer of gate material 262 can include 100 to200 nm of polysilicon and possibly a layer of metal silicide on thepolysilicon, such as 100 to 200 nm of tungsten silicide. Other materialswhich can be used to implement the layer of gate material 262 are withinthe scope of the instant example. Next, a patterned photoresist layer264 is conventionally formed over the layer of gate material 262.

As shown in FIG. 2E, after patterned photoresist layer 264 has beenformed, the exposed regions of the layer of gate material 262 are etchedin a conventional manner to expose gate dielectric layer 260 and form agate 270. Following the etch, patterned photoresist layer 264 is removedin a conventional fashion.

As shown in FIG. 2F, after patterned photoresist layer 264 has beenremoved, gate sidewall spacers 272 are conventionally formed on thelateral surfaces of the gates 270. The gate sidewall spacers 272 can beformed by forming a conformal layer of silicon dioxide 50 to 150 nmthick over the top surface of the semiconductor device, and thenremoving the silicon dioxide from horizontal surfaces using ananisotropic etch process, such as a reactive ion etch (RIE) process.

As further shown in FIG. 2F, a patterned photoresist layer 274 is nextconventionally formed on gate dielectric layer 260 and gate 270. Afterthis, dopants having the same conductivity type as drain drift region212 are implanted through patterned photoresist layer 274 to form asource region 280 and a drain region 282. Source region 280 reduces thesize of back gate region 234 and surface region 236. Drain region 282reduces the size of drain drift region 212.

Source region 280, which is heavily doped, touches back gate region 234and surface region 236. Drain region 282, which is also heavily doped,is formed between the STI regions 230 to touch drain drift region 212.Following the implant, patterned photoresist layer 274 is removed in aconventional manner. In the present example, the source and drainregions 280 and 282 are n+ regions. The implant can have a dose of8×10¹⁴ to 1×10¹⁶ and an energy of 20 keV to 70 keV.

As shown in FIG. 2G, after patterned photoresist layer 274 has beenremoved, a patterned photoresist layer 284 is next conventionally formedon gate dielectric layer 260 and gate 270. After this, dopants havingthe same conductivity type as back gate region 234 are implanted throughpatterned photoresist layer 284 to form a body contact region 286.

Body contact region 286, which is heavily doped, touches body region246. Following the implant, patterned photoresist layer 284 is removedin a conventional manner to complete the formation of a LDMOS transistorstructure 290. In the present example, body contact region 276 is a p+regions. The implant can have a dose of 8×10¹⁴ to 1×10¹⁶ and an energyof 20 keV to 70 keV.

It should be understood that the above descriptions are examples, andthat various alternatives described herein may be employed in variousimplementations. Thus, it is intended that the following claims definethe scope of the invention(s) and that structures and methods within thescope of these claims and their equivalents be covered thereby.

What is claimed is:
 1. An integrated circuit, comprising: asemiconductor substrate having a first conductivity type and a topsurface; a gate positioned above the top surface between a source regionand a drain region; a drain drift region having a second oppositeconductivity type extending from the drain region toward the sourceregion and located partially under the gate, the drain drift regionhaving: a first drain drift sub-region (DDSR) near the surface andextending toward the source region a first distance; a second DDSR belowthe first DDSR and extending toward the source region a second distanceless than the first distance; and a third DDSR between the first DDSRand the second DDSR and extending toward the source region a thirddistance greater than the second distance and less than the firstdistance.
 2. The integrated circuit of claim 1, wherein the drain driftregion further includes: a fourth DDSR between the second and the thirdDDSR and extending toward the source region by a fourth distance greaterthan the third distance and less than the first distance.
 3. Theintegrated circuit of claim 1, wherein the first DDSR has a dopantconcentration greater than the third dopant concentration.
 4. Theintegrated circuit of claim 1, wherein the first DDSR and the fourthDDSR have a dopant concentration greater than the second DDSR.
 5. Theintegrated circuit of claim 1, further comprising: a drain region abovethe first drain drift region and laterally spaced apart from the gate;and an isolation structure between the drain region and the gate.
 6. Theintegrated circuit of claim 1, wherein the drain drift region is n-typeand the substrate is p-type, further comprising: a first p-type regionextending from the source region toward the drain region by a fifthdistance and partially under the fourth DDSR; a second p-type regionlocated between the first p-type region and the surface and extendingfrom the source region toward the drain region and partially under thegate by a sixth distance less than the fifth distance.
 7. The integratedcircuit of claim 6, further comprising a third p-type region locatedunder the drain drift region and the first p-type region, the thirdp-type region having a p-type dopant concentration greater than that ofthe substrate.
 8. A transistor, comprising: a p-type substrate having atop surface and a p-buried layer (PBL); an n-type source region and ann-type drain region in the substrate and a gate electrode positionedbetween the source and drain regions; an n-type drain drift regionbetween the PBL and the drain region and extending under a first portionof the gate electrode; a p-type body region between the PBL and thesource region and extending under a second portion of the gateelectrode; and a p-type intermediate region between the PBL and thep-type body region, wherein: the p-type body region extends laterallyfrom under the source toward the drain by a first distance; the p-typeintermediate region extends laterally from under the source toward thedrain by a second distance greater than the first distance.
 9. Thetransistor of claim 8, wherein: the p-type body region has a first peakhorizontal dopant concentration at a first depth below the top surface,and the p-type intermediate region has a second peak horizontal dopantconcentration at a second depth greater than the first depth below thetop surface; the n-type drift region has a third horizontal peak dopantconcentration at a third depth below the top surface that is less thanthe first depth; and the n-type drift region has a fourth horizontalpeak dopant concentration at a fourth depth between the first and seconddepths.
 10. The transistor of claim 9, wherein the first horizontal peakdopant concentration is greater than the second horizontal peak dopantconcentration.
 11. The transistor of claim 8, further comprising anisolation structure between the gate electrode and the drain region. 12.The transistor of claim 8, wherein a portion of the n-type drift regionis located between the p-type intermediate region and the gateelectrode.
 13. The transistor of claim 9, wherein a portion of then-type drift region having the fourth horizontal peak concentration islocated vertically between the top surface and a portion of theintermediate region having the second horizontal peak concentration. 14.A method of forming an integrated circuit, comprising: forming a sourceregion and a drain region having a first conductivity type in asemiconductor substrate having a second opposite conductivity type;forming a gate over a top surface of the substrate between the sourceregion and the drain region; forming under the drain region andpartially under the gate a drain drift region having the firstconductivity type and extending from the drain region toward the sourceregion, the drain drift region having: a first drain drift sub-region(DDSR) near the surface and extending toward the source region a firstdistance; a second DDSR below the first DDSR and extending toward thesource region a second distance less than the first distance; and athird DDSR between the first DDSR and the second DDSR and extendingtoward the source region a third distance greater than the seconddistance and less than the first distance.
 15. The method of claim 14,wherein the drain drift region further includes: a fourth DDSR betweenthe second and the third DDSR and extending toward the source region bya fourth distance greater than the third distance and less than thefirst distance.
 16. The method of claim 14, wherein the first DDSR has adopant concentration greater than the third dopant concentration. 17.The method of claim 14, wherein the first DDSR and the fourth DDSR havea dopant concentration greater than the second DDSR.
 18. The method ofclaim 14, further comprising forming an isolation structure between thedrain region and the gate.
 19. The method of claim 14, wherein the draindrift region is n-type and the substrate is p-type, further comprising:forming under the source region a first p-type region extending from thesource region toward the drain region by a fifth distance and partiallyunder the fourth DDSR; forming a second p-type region located betweenthe first p-type region and the surface and extending from the sourceregion toward the drain region and partially under the gate by a sixthdistance less than the fifth distance.
 20. The method of claim 19,further comprising forming a third p-type region located under the draindrift region and the first p-type region, the third p-type region havinga p-type dopant concentration greater than that of the substrate.